The present invention relates to a method of and an apparatus for molding glass articles and, more particularly, to a method of molding glass articles having high configuration precision or accuracy and fine surface roughness so that grinding or polishing after press-molding can be dispensed with, and to an apparatus for carrying out the molding method. The method and apparatus are particularly suitable for molding glass lenses large in diameter and large in thickness.
In recent years, studies and researches have been conducted in which particular material is used to form a mold having an optical mirror surface, and pressmolding is carried out within non-oxidizing atmosphere, thereby obtaining lenses having an optical mirror surface necessitating no grinding or polishing after the press-molding. In this connection, U.S. Pat. No. 3,833,347 discloses the use of glassy carbon as the material of the mold. It is disclosed in U.S. Pat. No. 4,139,677 to employ SiC or Si.sub.3 N.sub.4 as the material of the mold. Further, it is disclosed in U.S. Pat. No. 4,168,961 to utilize mixture of SiC and carbon as the material of the mold. According to the molding method disclosed in these U.S. patents, a glass preform within the mold is first heated to a temperature level approximate to a glass softening point, that is, a Litolton point at which the temperature level corresponds to the viscosity of 10.sup.7.65 poises. A load is then applied to the glass preform within the mold, to carry out molding of the glass preform. Subsequently, the pressed glass is cooled to a glass transition temperature or below, while the load is maintained applied to the pressed glass in order to prevent the pressed glass from being deformed. After the pressed glass has been solidified completely, the load is removed from the pressed glass. Subsequently, the mold is cooled to a low temperature level of 300 degrees C., and then the mold is opened. It is required for such molding method to carry out the above various processings at the same position, resulting in such a problem that the molding cycle time is lengthened considerably.
On the other hand, Japanese Patent Application Laid-Open No. 58-84134 discloses an example of a glass article molding method which employs a glass preform having a configuration resembling a final product. In this method, the glass preform and a mold are heated to a temperature level corresponding to a glass viscosity level within a range of from about 10.sup.8 to about 10.sup.12 poises. A load is then applied to the glass preform to shape the same. Subsequently, the pressed glass and the mold are cooled. The load is then removed from the pressed glass, while the glass viscosity is maintained at a value lower than 10.sup.13 poises. The pressed glass is then taken out of the mold. Also in this conventional glass article molding method, the pressed glass is cooled within the mold while the load is maintained applied to the pressed glass, until the pressed glass is solidified, similarly to the above-described method disclosed in the U.S. patents. Accordingly, the time required for the pressing step is likewise lengthened, resulting in such a problem that it is impossible to expect a great improvement in the entire molding speed.
Another method of molding glass articles is disclosed in Japanese Patent Application Laid-Open No. 61-21927. In the method, a mold is employed, which is composed of a pair of cope and drag or upper and lower mold portions. The cope and the drag are fitted in a sleeve in such a manner that at least the cope is movable toward and away from the drag. A glass preform is inserted into the mold. A pressure rod separate from the cope applies a load to the cope at a temperature level which corresponds to the viscosity of the glass preform within a range of from 10.sup.8.5 to 10.sup.10.5 poises, to press the glass preform for few seconds to several tens of seconds, thereby molding the glass preform. The press rod is then moved backward to remove the load from the pressed glass. Subsequently, while the pressed glass is maintained accommodated in the mold, the pressed glass is cooled until the glass viscosity reaches a value equal to or higher than 10.sup.11.5 poises. The method is advantageous in that, since the mold having accommodated therein the pressed glass is cooled with the load removed after the press-molding, the press step and the cooling step can be carried out at their respective positions independent of each other, thereby making it possible to raise the entire molding speed. Since, however, the glass preform is pressed only once at the temperature corresponding to the glass viscosity within the range of from 10.sup.8.5 to 10.sup.10.5 poises, a difference in temperature occurs between the surface layer of the glass preform and the interior thereof at the cooling, particularly in case where an attempt is made to obtain a glass article which is large in size and large in thickness. This results in such a problem that recesses are generated in the molded glass by shrinkage thereof.